12
chloride salts, respectively (García et al. 2015). On another note, Florindo et al.
proved that there is no significant difference in density values whether the heating
method or the grinding method was used for the preparation of deep eutectic solvents (Florindo et al. 2014). Yet, differences of up to 4% were detected between the
available literature sources when it comes to the density of the most studied 1:2
ChCl:U deep eutectic solvent (García et al. 2015). A series of studies aiming to
efficiently predict the density of deep eutectic solvents were conducted by Mjalli
et al. via several theoretical approaches (Mjalli 2016; Mjalli et al. 2015; Shahbaz
et al. 2011a, 2013; Shahbaz et al. 2012b). The mass connectivity index-based correlation, taking into account the molecular structures of deep eutectic solvents’
forming compounds, allowed the prediction of the density of different type III deep
eutectic solvents as a function of temperature with a very high efficiency
(Mjalli 2016).
1.4.3 Viscosity
The viscosity is another important and extensively studied property of deep eutectic
solvents. Most of the reported deep eutectic solvents to date are highly viscous at
room temperature (ɳ > 100 mPa.s) which is mainly ascribed to the extensive hydrogen bond network taking place between deep eutectic solvents’ components. In
addition, they present a very broad viscosity range. In fact, ChCl:EG (1:2) is known
to have a very low viscosity (37 mPa.s at 25 °C), while sugar-based deep eutectic
solvents present extremely large viscosities (12,730 mPa.s for 1:1 ChCl:sorbitol at
30 °C and 34,400 mPa.s for 1:1 ChCl:glucose at 50 °C), and even higher viscosities
were recorded for metal salt-based deep eutectic solvents (85,000 mPa.s for 1:2
ChCl:zinc chloride at 25 °C) (Zhang et al. 2012). Yet, very low viscosities were
recorded for hydrophobic deep eutectic solvents based on DL-menthol (7.61 mPa.s
at 25 °C for 1:3 DL-menthol:octanoic acid) (Nunes et al. 2019; Ribeiro et al. 2015).
The viscosity of a eutectic mixture is clearly affected by the nature of its components (Abbott et al. 2007a; D’Agostino et al. 2011), their molar ratio (Abbott et al.
2011), the temperature (Abbott et al. 2004a; Abbott et al. 2003, 2006; Dai et al.
2015; Kareem et al. 2010), and the water content (D’Agostino et al. 2015; Dai et al.
2015; Du et al. 2016; Florindo et al. 2014; Shah and Mjalli 2014). The effect of
water will be discussed in detail in the upcoming sections. Moreover, the viscosity
not only depends on the intermolecular forces between the hydrogen bond donor
and the ion but also on the steric effects which can be quantified by the hole theory.
The latter considers the existence of holes or voids in the fluid which affects the
fluid’s viscosity and ionic conductivity (Abbott et al. 2006). The distribution of
holes of radius r is influenced by the hydrogen bond donor and the salt. It also seems
that deep eutectic solvents containing large holes are less viscous because they
allow a certain ionic motion (García et al. 2015). On a separate note, it is worthy to
mention that large differences were noticed when comparing the viscosity data
obtained by different researchers for the same deep eutectic solvent (e.g., 152 mPa.s
T. El Achkar et al.
chloride salts, respectively (García et al. 2015). On another note, Florindo et al.
proved that there is no significant difference in density values whether the heating
method or the grinding method was used for the preparation of deep eutectic solvents (Florindo et al. 2014). Yet, differences of up to 4% were detected between the
available literature sources when it comes to the density of the most studied 1:2
ChCl:U deep eutectic solvent (García et al. 2015). A series of studies aiming to
efficiently predict the density of deep eutectic solvents were conducted by Mjalli
et al. via several theoretical approaches (Mjalli 2016; Mjalli et al. 2015; Shahbaz
et al. 2011a, 2013; Shahbaz et al. 2012b). The mass connectivity index-based correlation, taking into account the molecular structures of deep eutectic solvents’
forming compounds, allowed the prediction of the density of different type III deep
eutectic solvents as a function of temperature with a very high efficiency
(Mjalli 2016).
1.4.3 Viscosity
The viscosity is another important and extensively studied property of deep eutectic
solvents. Most of the reported deep eutectic solvents to date are highly viscous at
room temperature (ɳ > 100 mPa.s) which is mainly ascribed to the extensive hydrogen bond network taking place between deep eutectic solvents’ components. In
addition, they present a very broad viscosity range. In fact, ChCl:EG (1:2) is known
to have a very low viscosity (37 mPa.s at 25 °C), while sugar-based deep eutectic
solvents present extremely large viscosities (12,730 mPa.s for 1:1 ChCl:sorbitol at
30 °C and 34,400 mPa.s for 1:1 ChCl:glucose at 50 °C), and even higher viscosities
were recorded for metal salt-based deep eutectic solvents (85,000 mPa.s for 1:2
ChCl:zinc chloride at 25 °C) (Zhang et al. 2012). Yet, very low viscosities were
recorded for hydrophobic deep eutectic solvents based on DL-menthol (7.61 mPa.s
at 25 °C for 1:3 DL-menthol:octanoic acid) (Nunes et al. 2019; Ribeiro et al. 2015).
The viscosity of a eutectic mixture is clearly affected by the nature of its components (Abbott et al. 2007a; D’Agostino et al. 2011), their molar ratio (Abbott et al.
2011), the temperature (Abbott et al. 2004a; Abbott et al. 2003, 2006; Dai et al.
2015; Kareem et al. 2010), and the water content (D’Agostino et al. 2015; Dai et al.
2015; Du et al. 2016; Florindo et al. 2014; Shah and Mjalli 2014). The effect of
water will be discussed in detail in the upcoming sections. Moreover, the viscosity
not only depends on the intermolecular forces between the hydrogen bond donor
and the ion but also on the steric effects which can be quantified by the hole theory.
The latter considers the existence of holes or voids in the fluid which affects the
fluid’s viscosity and ionic conductivity (Abbott et al. 2006). The distribution of
holes of radius r is influenced by the hydrogen bond donor and the salt. It also seems
that deep eutectic solvents containing large holes are less viscous because they
allow a certain ionic motion (García et al. 2015). On a separate note, it is worthy to
mention that large differences were noticed when comparing the viscosity data
obtained by different researchers for the same deep eutectic solvent (e.g., 152 mPa.s
T. El Achkar et al.